Numerous processes are known for producing spinnable solutions of polymerization products, such as polyacrylonitrile for example, for the production of synthetic filaments and fibers. For example, acrylonitrile polymers may be dissolved with stirring in dimethyl formamide or another polar organic solvent, such as dimethyl acetamide, dimethyl sulphoxide, etc., at a temperature of from 70.degree. to 90.degree. C., followed by filtering the resulting solution and spinning it into filaments. In this case, large vessels are generally used for technical reasons, with the result that the natural color of the spinning solutions is spoiled in the event of prolonged spinning times. In cases where less soluble polymers, for example polymers having a high K-value, or special polymers without any plasticizing comonomer components, such as acrylonitrile homopolymers, for example, are used, the dissolution temperatures generally have to be considerably increased in order to obtain spinnable, lump-free and gel-free and, above all, viscosity-stable solutions. This additional temperature burden on the spinning solution gives rise to serious problems in regard to its natural color and the retention thereof over a prolonged spinning period. Accordingly, there has been no shortage of proposals to overcome these problems. Broadly speaking, these proposals may be divided into two groups. The first group is based on additions to the spinning solution which positively affect its natural color, viscosity behavior and spinnability. The other group is based on special procedures for preparing homogeneous spinning solutions.
Of this latter group, particular emphasis is placed on German Patent Application No. P 41 52 29b, 3/65 which describes a process in which the size-reduced polymer is suspended in solvents and, after transfer to a heating zone, the resulting suspension is heated to a temperature (for example 150.degree. C.) at which the solvent has a marked dissolving effect on the polymer and the clear colorless solution formed is spun into filaments. In this process, the solution formed in the heating zone is not "kept for any significant time at an elevated temperature" in order to prevent the color of the solution from being damaged. According to claim 2 and the Examples, this is understood to be a period of around 40 seconds.
Tests carried out with difficulty soluble polymers have shown that, although this process of suspending the polymer and subsequently transferring it to a heating zone is still the most suitable, the improvement in the natural color of the spinning solution is again offset by numerous spinning problems in the form of variations in denier and tackiness of the filaments. Thus, the use of this material, for example in the rayon sector, with its numerous texturing steps would give rise to difficulties. Capillary breaks, rucking and cracks occur to an increased extent.